Download Exam 1 (Chapters 1-4) - UNC Physics and Astronomy

Phys25, Summer 2006, Exam 1
Student Name: ___________________________ Seat Number: ________
Sequence Number: 101
Honor Pledge and signature:
I have neither given nor received unauthorized aid on this examination. _____________________
Instructions:
This exam is closed book, closed notes. However, you may use a calculator.
Mark your answers to the multiple-choice questions on a Scantron answer sheet.
Show all of your work on these test papers for full credit (no other scratch paper is allowed).
Questions 1-18 are worth 2 points each, while questions 19-31 are 3 points each.
Equations and conversion factors that may be useful:

 F
q
q
V
E
Ek 2
E
  EA cos   encl
qo
s
o
r
F  qE
I
F k
Q
t
q1 q 2
r2
V  IR
Q
V
L
R
A
C
Req  R1  R2  R3  
1 Req 1 R1  1 R2  
F  qvB sin 
F
 o I1 I 2
L
2d
  N
x rms 

t
1
2
x max
I S VP N P


I P VS
NS
 o A
d
P  IV
u E  12  o E 2
uB 
B2
2 μo
o I
2r
Bloop 
2R
V 
kq
r
U  12 CV 2
U  12 LI 2
q (t )  Qe t / 
  RC
  BA cos 
 L R
1 C eq 1 C1  1 C 2  
  NIAB sin 
N o I
U
qo
q (t )  Q(1  e t /  )
C eq  C1  C 2  C 3  
F  ILB sin 
Bwire 
C
V 
Bsolenoid   o nI
L   o n 2 Al
I

 rod  Bvl sin    NBA sin t I  (1  e t /  )
t
R
V
1
I rms  rms Z  R 2  ( X L  X C ) 2 X C 
X L  L
Z
C
1
o 
 2f Pav  I rmsVrms cos 
1 G  10 -4 T
LC
  L
 o  8.85  10 12 C 2 /(N  m 2 )
e  1.602  10 19 C
k
1
4o
 o  4  10 7 T  m/A
mp  1.67  10  27 kg
 8.99  10 9 N  m 2 /C 2
me  9.11  10 31 kg
E max, air  3  10 6 V/m
1. An electron and proton are separated by 1.0 m at a height of 1.0 m above the surface of the Earth, which
has an electric field of 100 N/C pointing down. Where could we expect to find these particles 2.0 s
after they are released?
1. The particles will collide and land on the ground below the proton's original position.
2. The particles will collide and land on the ground midway between their original positions.
3. The proton will be on the ground, while the electron will be out in space.
4. The electron will be on the ground, while the proton will be out in space.
2. In the expression Q = CV, Q is the total charge on the plates of a capacitor.
1. True
2. False
3. A metal sphere of radius 5 cm has a charge of +100 μC. A second metal sphere with charge of +50 μC
and a radius of 10 cm is brought in contact with the first sphere and then separated. What is the charge
on each sphere?
1. +100 μC on the larger sphere, +50 μC on the smaller sphere
2. +50 μC on the larger sphere, +100 μC on the smaller sphere
3. +75 μC on each sphere
4. More information is needed to answer this question.
4. A proton is released in a region with an electric field. As the proton moves in response to this field,
1. its electrical potential increases while its electrical potential energy decreases.
2. its electrical potential decreases while its electrical potential energy increases.
3. both its electrical potential and electrical potential energy increase.
4. both its electrical potential and electrical potential energy decrease.
5. Three point charges are arranged on the corners of a square as shown (clockwise starting from upper
right: zero, +Q, -2Q, +Q). What charge must be placed at the empty corner of the square so that both
the electric field and electric potential are zero at the center?
+Q
d
1. -Q
2. +2Q
d
3. -2Q
4. not possible
-2Q
+Q
6. Adjacent coils in a solenoid experience a force of attraction as a result of the current that flows through
them.
1. True
2. False
7. The capacitance of a parallel-plate capacitor depends on all of the following quantities, except:
1. The area of the plates
2. The separation between the plates
3. The material between the plates
4. The amount of charge on the plates
8. What do capacitors in series have in common?
1. same voltage
2. same capacitance
3. same charge on each plate
4. same magnitude of charge on each plate
9. Kirchhoff's junction rule essentially states that charge is conserved in a circuit.
1. True
2. False
10. If an electron moves with constant velocity in a region with no magnetic fields, then we can conclude
that E = 0.
1. True
2. False
11. If an electron moves upward (toward the sky) near the Earth's equator, which direction is this particle
deflected?
1. north
2. south
3. east
4. west
2
12. A gold bracelet lies flat on a horizontal surface. A bar magnet is held above the center of the coil with
its south pole pointing up. If the magnet is dropped, what is the direction of the induced current as
viewed from above the coil?
1. clockwise
2. counterclockwise
3. There is no current induced in the coil.
4. There is not enough information to determine the direction.
13. When does an incandescent light bulb have the greatest current flowing through it?
1. right after the light is turned on
2. a few seconds after the light is turned on
3. a long time after the light is turned on
14. Consider an AC circuit where an inductor is in series with an incandescent light bulb. If the frequency
of the generator is decreased, what will happen to the brightness of the bulb?
1. The bulb will stay the same brightness.
2. The bulb will shine brighter.
3. The bulb will become dimmer.
15. A long wire of resistance R is wound into a coil with many loops. If this wire is connected to an AC
generator, how does the current through the straight wire compare with the current through the coiled
wire?
1. I(straight) = I(coil)
2. I(straight) < I(coil)
3. I(straight) > I(coil)
16. In an RLC circuit, what is the phase angle between the voltages across the inductor and capacitor?
1. 0º
2. 45º
3. 90º
4. 180º
17. The primary coil of a transformer has 100 turns and its secondary coil has 400 turns. If the ac current
in the primary coil is 2 A, what is the current in its secondary coil?
1. 0.5 A
2. 2 A
3. 4 A
4. 8 A
18. Two wires lie in a horizontal plane but are insulated from each other. One wire carries a current of 6 A
directed north, while the other wire has a current of 8 A toward the east. What is the general direction
of the magnetic field due to these currents at a point directly above their intersection?
1. northeast
2. northwest
3. southeast
4. southwest
19. Two wires lie in a horizontal plane but are insulated from each other. One wire carries a current of 6 A
directed north, while the other wire has a current of 8 A toward the east. What is the magnitude of the
magnetic field due to these currents at a point 0.5 m above their intersection?
3
1.
2.
3.
4.
4 μT
6 μT
8 μT
40 μT
20. How far above a proton would an electron have to be held to exert an electrostatic force that just equals
the weight of the proton?
1. 0.12 m
2. 5.1 m
3. 12 m
4. 51 m
21. A metal sphere of radius r = 4.0 cm has a charge of +12 μC. What is the force on a point charge of
-6 μC located 15 cm from the surface of the sphere?
1. 18 N toward the sphere
2. 29 N toward the sphere
3. 29 x 10-3 N toward the sphere
4. 4.3 N away from the sphere
22. How much work is required to assemble four point charges, each with q = -8 pC, from infinity to a
distance of 2 nm from each other?
1. -1.2 mJ
2. +1.2 mJ
3. -1.7 mJ
4. +1.7 mJ
23. A parallel-plate capacitor has plates each with an area of 55 cm2 separated by 0.40 mm of paper (with
dielectric constant 3.7). What is the capacitance?
1. 0.45 nF
2. 4.5 μF
3. 45 nF
4. 66 nF
24. A Van de Graaff generator produces sparks up to 10 cm long between the charged dome and a
grounded sphere. Estimate the electrical potential of the dome relative to ground.
1. 300 V
2. 3 kV
3. 30 kV
4. 300 kV
25. Three 600 Ω resistors are connected to form a triangle. What is the resistance between any two
vertices of the triangle?
1. 200 Ω
2. 400 Ω
3. 900 Ω
4. 1800 Ω
4
26. What would be the combined power consumed by two 100-W light bulbs connected in series to a
120-V power supply?
1. 200 W
2. 100 W
3. 50 W
4. 25 W
27. A gold necklace has a resistance of 0.0042 Ω and forms a loop with an area of 450 cm2. There is a
magnetic field of 2.0 T perpendicular to the plane of the necklace, and the field is changing at the rate
of 30 T/s. What is the current induced in the necklace?
1. 6.5 A
2. 320 A
3. 640 A
4. 3350A
28. Find the magnitude of the force acting on a proton moving eastward with a speed of 8.0 km/s where the
only magnetic field is that due to the Earth, which is 0.6 G and points north and 55 degrees below
horizontal at this location.
1. 6.310-16 N
2. 6.310-20 N
3. 7.710-20 N
4. 4.410-20 N
29. A metal Slinky with a diameter of 7.5 cm is stretched to a length of 37 cm. If a current of 2.1 A is sent
through the 56 coils of this Slinky, what is the magnetic field at the center of this solenoid?
1. 1.5 G
2. 2.0 G
3. 20 G
4. 4.0 G
30. A series RLC circuit has a 100-Ω resistor, a 0.20-µF capacitor, and a 4.0-mH inductor connected
across a 120-V rms ac generator operating at 60 Hz. What is the resonant frequency of this circuit?
1. 60 Hz
2. 5.6 kHz
3. 11 kHz
4. 35 kHz
31. An rms voltage of 120 V at 60 Hz is applied across a capacitor and a 100-ohm resistor. If the
impedance of this circuit is 200 ohms, what is the average power of this circuit?
1. 36 W
2. 72 W
3. 144 W
4. 280 W
5